A lithium ion battery has been conventionally used as a power supply for a portable electronic apparatus. The lithium ion battery is generally configured by disposing a positive electrode, a negative electrode, and a separator in an electrolyte solution. In the positive electrode, lithium cobaltate or lithium manganate is applied to the surface of an aluminum foil. In the negative electrode, carbon is applied to the surface of a copper foil. The separator is disposed so as to separate the positive electrode and the negative electrode, to prevent a short circuit between the positive electrode and the negative electrode.
When the lithium ion battery is charged, lithium ions are released from the positive electrode and move to the negative electrode. In contrast, when the lithium ion battery is discharged, lithium ions are released from the negative electrode and move to the positive electrode. Such charge and discharge are repeated in the lithium ion battery. Therefore, it is necessary that the separator used in the lithium ion battery can allow lithium ions to well permeate there.
Various polypropylene porous films having a high heat resistance have been proposed as the separator. For example, Patent Literature 1 has proposed a method for producing a polypropylene microporous film. The method includes melting and extruding a composition containing polypropylene, a polymer having a melt crystallization temperature higher than that of polypropylene, and a β-crystal nucleating agent into a sheet, and stretching the sheet at least uniaxially.
When the lithium ion battery is repeatedly charged and discharged, the electrolyte solution is subjected to reductive decomposition and oxidative decomposition in the negative electrode or the positive electrode. As a result, a portion where the electrolyte solution is not present is generated in an active material layer of the negative electrode or the positive electrode. Such a portion inhibits the diffusion of lithium ions, and therefore the discharge capacity of the lithium ion battery rapidly decreases. Such a phenomenon is generally called a “liquid shortage.”
Therefore, it is desired to improve electrolyte solution retention property of a polypropylene microporous film, in order to store a sufficient amount of electrolyte solution in a battery and to reduce the occurrence of liquid shortage phenomenon.
However, the polypropylene microporous film obtained by the method of Patent Literature 1 has the low electrolyte solution retention property and cannot sufficiently reduce a decrease in discharge capacity caused by liquid shortage of the lithium ion battery.